The capacity of optical communications is progressively increased, then increasing the transmission capacity by a wavelength division multiplexing (WDM) technology. On the other hand, an increase in throughput of the path switching function in a node is strongly desired. In a mainstream method of path switching of wavelength multiplexed channels in the related art, the path switching is performed by use of an electric switch after an optical signal corresponding to each of the transmitted channels has been converted to an electric signal. However, by making full use of the features of an optical signal which is fast and has a wideband, ROADM (Reconfigurable optical add/drop multiplexer) systems are introduced, in which adding, dropping and/or the like are performed with an optical switch and the like for the optical signal corresponding to each channel without OE (optical-to-electrical) and EO (electrical-to-optical) conversion. Specifically, as a network node of an optical ring network, node equipment is provided to add/drop the optical signal corresponding to each channel and to pass the optical signal that does not require a drop operation without OE and EO conversion. The node equipment which performs adding, dropping and/or the like on an optical signal corresponding to each channel without OE and EO conversion has the advantages of small size and low power consumption. A wavelength selective switch module is desired as a device required for the future development of these ROADM systems.
In particular, the need for the multi-input multi-output wavelength selective switch as optical switches interconnecting a plurality of ROADM systems is increasing. This corresponds to a ROADM configuration called CDC (Colorless Directionless Contentionless), which is expected to be applied to next generation networks. In response to such demands, an M×N wavelength selective switch having an MEMS (Mechanical-Electro Machine System) is known (see, for example, PTL 1). The wavelength selective switch having a LCOS (Liquid Crystal On Silicon) is allowed to flexibly re-locate a wavelength grid, increasing the wavelength use efficiency.
FIG. 18 is a schematic diagram of a wavelength selective switch disclosed in PTL 1. In FIG. 18, an optical signal incident from an input optical fiber 101 is converted into parallel light by a convex lens 103, then is demultiplexed into wavelengths by a diffraction grating 104, and then focused onto a MEMS mirror array 106a by a convex lens 105. The optical signal is reflected for each wavelength by the MEMS mirror array 106a to be reflected by a total reflection mirror 402 via a convex lens 401. The optical signal thus reflected is incident on a MEMS mirror array 106b via the convex lens 401 again, so that the optical path is changed, thus outputting the optical signal to an output optical fiber 102 through the convex lens 105, the diffraction grating 104 and the convex lens 103. FIG. 19 is a diagrammatic illustration of a light beam in a switch axis direction. In the configuration in FIG. 19, the same 4f system (f is a focal length of the convex lens 401) is set between the MEMS mirror arrays 106a and 160b and the total reflection mirror 402 in both a wavelength dispersion axis direction of the diffraction grating 104 and a switch axis direction perpendicular to the wavelength dispersion axis direction. The reason why the 4f system is set is that a position of a beam waist is determined at both the MEMS mirror arrays 106a and 106b as parallel beams in both the wavelength dispersion axis direction and the switch direction on the reflection mirror 402. Such an optical system, even if two mirror elements 107a and 107b are tilted at any angle, the light absolutely returns to the mirror element 107, making switching impossible. To solve this problem, the wavelength selective switch disclosed in PTL 1 involves the necessity to form the reflection mirror 402 in a blazed shape 1200 that is complicated and has poor productivity, as shown in FIG. 20. As shown in FIG. 21, there is considered a method of forming the reflection mirror 402 in a curved shape to implement switching. However, the advantages of adopting the foregoing 4f system (point of forming the beam waist on the MEMS mirror arrays 106a and 106b) are traded off for this method.
The configuration of the wavelength selective switch illustrated in FIG. 18, FIG. 19 and FIG. 20 has the advantages that:
(1) components can be set such that the position of the beam waist is to be formed on the MEMS mirror arrays 106a and 106b for an increase in the reflection efficiency; and
(2) the MEMS mirror arrays 106a and 106b can be placed on the same plane.
FIG. 22 is a diagram showing a schematic configuration of a wavelength selective switch disclosed in PTL 1 as well. In FIG. 22, optical signals from input optical fibers 101 enter the MEMS mirror array 106a via spherical lenses 601 placed for individual input ports and cylindrical lenses 602. The optical signal after the optical path has been changed by the MEMS mirror array 106a is reflected through a convex lens 603 by a total reflection mirror 604 as in the case of the description with reference to FIG. 18. The optical signal reflected by the total reflection mirror 604 passes through the convex lens 603, the MEMS mirror array 106b, the cylindrical lens 602 and the spherical lens 601 to be coupled to the output optical fiber 102 again.